1. Field
This invention relates to wastewater treatment methods for increasing denitrification rates of aerobic and anaerobic digesters. More particularly, it comprises a method of conditioning, separating and comminuting sulfurous acid treated suspended solids for addition to aerobic and anaerobic digesters to provide electron donor carbon and sulfur compounds to increase the removal rate of ammonia, nitrates/nitrites, and BOD compounds.
2. State of the Art
Various types of wastewaters are known. As used herein, it is principally directed to treat wastewater process streams containing organic and macronutrients having already undergone primary and secondary treatment according to conventional wastewater treatment plant processes.
One source of wastewater is that present in sewage treatment gathering systems, which are processed by various methods. Most large municipal systems employ a series of settling ponds sequentially concentrating the solids contained in wastewater either with or without polymers for separation from liquids via mechanical separation means, such as belt presses. To produce a clean effluent that can be safely discharged to watercourses, wastewater treatment operations use three or four distinct stages of treatment to remove harmful contaminants; according to the United Nations Environmental Programme Division of Technology, Industry, and Economics Newsletter and Technical Publications Freshwater Management Series No. 1, “Bio-solids Management: An Environmentally Sound Approach for Managing Sewage Treatment Plant Sludge”. 
Preliminary wastewater treatment usually involves gravity sedimentation of screened wastewater to remove settled solids. Half of the solids suspended in wastewater are removed through primary treatment. The residual material from this process is a concentrated suspension called primary sludge, subsequently undergoing additional treatment to become bio-solids.
Secondary wastewater treatment is accomplished through a biological process, removing biodegradable material. This treatment process uses microorganisms to consume dissolved and suspended organic matter, producing carbon dioxide and other by-products. The organic matter benefits by providing nutrients needed to sustain the communities of microorganisms. As microorganisms feed, their density increases and they settle to the bottom of processing tanks, separated from the clarified water as a concentrated suspension called secondary sludge, biological sludge, waste activated sludge, or trickling filter humus. By breaking down the sludge, the wastewater system loses energy and increases carbon dioxide emissions.
Tertiary or advanced treatment is used when extremely high-quality effluent is required, including direct discharge to a drinking water source. The solid residual collected through tertiary treatment consists mainly of chemicals added to clean the final effluent, which are reclaimed before discharge, and therefore not incorporated into bio-solids. Tertiary or advanced treatment does not reduce the treated wastewater brine content, requiring energy intensive Quaternary brine treatment removal using reverse osmosis and distillation, and other methods.
Combined primary and secondary solids comprise the majority of material used at municipal plants for bio-solids production. Careful management throughout the entire treatment process allows plant operators to control the solids content, nutrient value and other constituents of bio-solids.
Biological treatment is used to remove ammonia through bacterial degradation. Wikipedia explains that
“Denitrification is a microbially facilitated process of nitrate reduction that may ultimately produce molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. This respiratory process reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter. The preferred nitrogen electron acceptors in order of most to least thermodynamically favorable include nitrate (NO3−), nitrite (NO2−), nitric oxide (NO), and nitrous oxide (N2O). In terms of the general nitrogen cycle, denitrification completes the cycle by returning N2 to the atmosphere. The process is performed primarily by heterotrophic bacteria (such as Paracoccus denitrificans and various pseudomonads), although autotrophic denitrifiers have also been identified (e.g., Thiobacillus denitrificans). Denitrifiers are represented in all main phylogenetic groups. Generally several species of bacteria are involved in the complete reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway has been identified in the reduction process.
Direct reduction from nitrate to ammonium, a process known as dissimilatory nitrate reduction to ammonium or DNRA, is also possible for organisms that have the nrf-gene. This is less common than denitrification in most ecosystems as a means of nitrate reduction. Other genes known in microorganisms which denitrify include nir (nitrite reductase) and nos (nitrous oxide reductase) among others; organisms identified as having these genes include Alcaligenes faecalis, Alcaligenes xylosoxidans, many in the Pseudomonas genus, Bradyrhizobium japonicum, and Blastobacter denitrificans. 
. . . Denitrification takes place under special conditions in both terrestrial and marine ecosystems. In general, it occurs where oxygen, a more energetically favorable electron acceptor, is depleted, and bacteria respire nitrate as a substitute terminal electron acceptor. Due to the high concentration of oxygen in our atmosphere, denitrification only takes place in environments where oxygen consumption exceeds the rate of oxygen supply, such as in some soils and groundwater, wetlands, poorly ventilated corners of the ocean, and in seafloor sediments.
Denitrification generally proceeds through some combination of the following intermediate forms:NO3−→NO2−→NO+N2O→N2(g)
The complete denitrification process can be expressed as a redox reaction:2NO3−10e−+12H+→N2+6H2O
The present invention described below proposes to drive this reaction using sulfurous acid treated solids producing sulfites and bisulfites and carbon as electron donors, while providing a growth substrate for certain biofilm bacteria, such as Nitrosomonas and Nitrobacter bacteria.
“ , , , Denitrification is commonly used to remove nitrogen from sewage and municipal wastewater.
Reduction under anoxic conditions can also occur through process called anaerobic ammonia oxidation (anammox):NH4++NO2−→N2+2H2O
“Carbon metabolism in bacteria is either heterotrophic, where organic carbon compounds are used as carbon sources, or autotrophic, meaning that cellular carbon is obtained by fixing carbon dioxide. Heterotrophic bacteria include parasitic types. Typical autotrophic bacteria are phototrophic cyanobacteria, green sulfur-bacteria and some purple bacteria, but also many chemolithotrophic species, such as nitrifying or sulfur-oxidising bacteria. Energy metabolism of bacteria is either based on phototrophy, the use of light through photosynthesis, or on chemotrophy, and the use of chemical substances for energy, which are mostly oxidised at the expense of oxygen or alternative electron acceptors (aerobic/anaerobic respiration).
Finally, bacteria are further divided into lithotrophs that use inorganic electron donors and organotrophs that use organic compounds as electron donors. Chemotrophic organisms use the respective electron donors for energy conservation (by aerobic/anaerobic respiration or fermentation) and biosynthetic reactions (e.g. carbon dioxide fixation), whereas phototrophic organisms use them only for biosynthetic purposes. Respiratory organisms use chemical compounds as a source of energy by taking electrons from the reduced substrate and transferring them to a terminal electron acceptor in a redox reaction. This reaction releases energy that can be used to synthesize ATP and drive metabolism. In aerobic organisms, oxygen is used as the electron acceptor. In anaerobic organisms other inorganic compounds, such as nitrate, sulfate or carbon dioxide are used as electron acceptors. This leads to the ecologically important processes of denitrification, sulfate reduction and acetogenesis, respectively.”; see Wikipedia, “Bacteria”//en.wikipedia.org/wiki/Bacteria.
As stated in Wikipedia, “Bacterial Growth”, //en.wikipedia.org/wiki/Bacterial_growth, bacterial growth to consume nutrients is a binary fission process with four different phases:
“In autecological studies, bacterial growth in batch culture can be modeled with four different phases: lag phase (A), exponential or log phase (B), stationary phase (C), and death phase (D):
1. During lag phase, bacteria adapt themselves to growth conditions. It is the period where the individual bacteria are maturing and not yet able to divide. During the lag phase of the bacterial growth cycle, synthesis of RNA, enzymes and other molecules occurs. So in this phase the microorganisms are not dormant.
2. Exponential phase (sometimes called the log phase or the logarithmic phase) is a period characterized by cell doubling. The number of new bacteria appearing per unit time is proportional to the present population. If growth is not limited, doubling will continue at a constant rate so both the number of cells and the rate of population increase doubles with each consecutive time period. For this type of exponential growth, plotting the natural logarithm of cell number against time produces a straight line. The slope of this line is the specific growth rate of the organism, which is a measure of the number of divisions per cell per unit time. The actual rate of this growth (i.e. the slope of the line in the figure) depends upon the growth conditions, which affect the frequency of cell division events and the probability of both daughter cells surviving. Under controlled conditions, cyanobacteria can double their population four times a day. Exponential growth cannot continue indefinitely, however, because the medium is soon depleted of nutrients and enriched with wastes.
3. During stationary phase, the growth rate slows as a result of nutrient depletion and accumulation of toxic products. This phase is reached as the bacteria begin to exhaust the resources that are available to them. This phase is a constant value as the rate of bacterial growth is equal to the rate of bacterial death.
4. At death phase, bacteria run out of nutrients and die.
This basic batch culture growth model draws out and emphasizes aspects of bacterial growth which may differ from the growth of macrofauna. It emphasizes clonality, asexual binary division, the short development time relative to replication itself, the seemingly low death rate, the need to move from a dormant state to a reproductive state or to condition the media, and finally, the tendency of lab adapted strains to exhaust their nutrients.
In reality, even in batch culture, the four phases are not well defined. The cells do not reproduce in synchrony without explicit and continual prompting (as in experiments with stalked bacteria) and their exponential phase growth is often not ever a constant rate, but instead a slowly decaying rate, a constant stochastic response to pressures both to reproduce and to go dormant in the face of declining nutrient concentrations and increasing waste concentrations.
To accelerate denitrification, it is therefore necessary to continually supply growth nutrients for denitrifying bacteria. In some wastewater treatment plants, small amounts of methanol, ethanol, acetate or proprietary products like MicroCg or MicroCglycerin are added to the wastewater to provide a carbon source for the denitrification bacteria. Methanol (CH3OH) serves as a carbon source for bacterial microbes Accelerated by the addition of methanol, anaerobic bacteria convert the nitrate to nitrogen gas, which is vented into the atmosphere.
Inventors Charles B. Bott, Ph.D. and Professor Robert Nerenberg propose to drive anoxic denitrification reactions using sulfurous acid producing sulfites and bisulfites as electron donors.
Today, wastewater treatment plants around the United States are using methanol in their denitrification process. One of the larger plants in the country, the Blue Plains Wastewater Treatment Facility, has had one of the best success stories related to methanol denitrification. Blue Plains, which serves the metropolitan Washington, D.C. area, releases nearly 350 million gallons of treated wastewater to the Potomac River each day. The Potomac flows into the Chesapeake Bay, the largest estuary in the United States. As a result of its size, the Blue Plains Wastewater Treatment Facility is the single largest point source of nitrogen for the Bay, at 20 tons of nitrogen per day. Methanol denitrification helped to reduce that number to 10 tons per day, half its original nitrogen discharge. The use of methanol denitrification at Blue Plains has resulted in a 30% drop in nitrogen levels in the Chesapeake Bay, from just one treatment plant.
Unfortunately, methanol, ethanol, and acetate are expensive. For example, methanol denitrification costs about $0.50 to $0.60 per pound of nitrogen removed for Blue Plains. Average nitrogen removal costs in the Chesapeake basin have been reported to be about $4 per pound, according to EPA Chesapeake Bay Program officials; see Methanol Institute; Chesapeake Bay Foundation, www.methanol.org.
Thus, there remains a need for a method to chemically treat wastewater undergoing bacterial denitrification with an inexpensive carbon and sulfur electron donor compound to expedite denitrification. The treatment method described below provides such an invention.